Electrical power is an important commodity in nearly all aspects of life. One such area where electrical power is important is in the hydrocarbon production arena. In particular, a number of large machines used in hydrocarbon production (pumps, etc.) require large amounts of electrical power. In many situations it is advantageous to be able to provide power locally, rather than having to lay power lines over vast distances. In particular, having to lay power lines may be problematic when the platform is offshore. As hydrocarbon production facilities move further and further away from land, this problem continues to grow in importance.
In addition, equipment used in the wellbore often requires a downhole electric power source for various operations. For example, a power source is required to operate circuits and instruments that measure the production capabilities of a well. In the prior art, batteries are typically used to provide the necessary power in a well.
However, a significant problem with the use of batteries as a power supply in a downhole environment is their limited life expectancy. The expected life of a battery is typically much shorter than in “standard” operating conditions due to harsh environmental factors. For example, the maximum temperature at which a battery performs reliably may be 200° C. Often, temperatures in the wellbore exceed this temperature. Further, caustic chemicals and fluids can cause early battery failure, causing delays and additional expense.
If a battery required by the downhole power source for the downhole instrument fails prior to the completion of testing, the testing string must be pulled back to the surface and the battery replaced or valuable rig time may be wasted on testing with no recorded data. This procedure causes an interruption in the collection of data as well as delay and further expense.
In addition, batteries can only provide a limited amount of power even when fully charged. This limits the use of circuits requiring higher power in a downhole environment.
For example, one operation which is often performed during the course of producing an oil or gas well is to lower a testing string into the wellbore to test the production capabilities of hydrocarbon producing underground formations intersected by the well. Testing is typically accomplished by lowering a string into the wellbore with an isolation packer attached to the string at its lower end. Once the test string is lowered to the desired final position, the isolation packer is set to seal off the annulus between the test string and the wellbore or casing, and small amounts of hydrocarbons are allowed to enter the test string. Measurements of pressure and temperature are then taken by transducers within the string, and the data is either stored in a memory, such as a random access memory (RAM) chip, within the testing circuit or transmitted to the surface. Depending on the data required for well test objectives, testing may last anywhere from one day to two weeks.
In another example, some production operations require the use of an electrical submersible pump (ESP). The ESP pumps downhole liquids and/or gases. The ESP requires a power source to operate. The ESP may receive power through an umbilical power cable connected to a surface power source. The amount of power provided by the umbilical power cable may be limited.
Accordingly, a need has arisen in the industry for a method and apparatus to extend the life of a downhole power source and/or increase the amount of power output from a downhole power source.
This disclosure incorporates by reference U.S. Pat. No. 5,149,984 and U.S. Pat. No. 5,965,964. In U.S. Pat. No. 5,149,984, a downhole electrical power supply apparatus uses a high fluid pressure source, such as a nitrogen chamber, to power a gas turbine. The rotational mechanical output of the gas turbine is converted to electrical power. The nitrogen chamber has a finite capacity with which to provide a high fluid pressure source to the gas turbine.
In U.S. Pat. No. 5,965,964, a piston is slidably attached to a sleeve and oscillates relative to the sleeve and a housing. The oscillation of the piston causes the oscillation of magnets relative to a wire coil section that generates electric power. Application of fluid pressure provided by drilling fluids during a drilling operation causes the piston to oscillate.
The above described current generator is available only during drilling operations. Accordingly, a method and apparatus for a downhole power source that is available during production and does not have a limited source of power is needed.